Fatigue resistant fasteners

ABSTRACT

A flush head fastener pin for use in aircraft structural joints which are countersunk for surface flushness. The pin is shaped and adapted to preload the joint material surrounding the countersink to create a predetermined residual stress pattern in that region for improvement of fatigue life. Preferred embodiments of a straight shank bolt, a tapered shank bolt, and a rivet are presented. Each pin embodiment has a 70* conical head and a concave transition portion of specified radius and smoothness interconnecting the head with the shank. The 70* head and transition portion cooperate to conformably deform and preload the countersunk region of the hole, which is preferably provided with a convex surface of specified radius at the base of the countersink, and a conical surface having a greater cone angle than that of the pin. The straight shank bolt embodiment further discloses a hardened convex lead-in portion of specified shape and smoothness extending between the shank and the threads to prevent galling of the hole during installation of the bolt.

United States Patent [191 Schmitt 5] July 2, 1974 FATIGUE RESISTANTFASTENERS Primary Examiner-H. Hampton Hunter [75] Inventor: Hubert A.Schmitt, Auburn, Wash. fg g g g or Flrm Bemard Donahue; [73] Assignee:The Boeing Company, Seattle,

Wash [57 ABSTRACT [22] Flled: May 1973 A flush head fastener pin for usein aircraft structural [21] Appl. No.: 356,877 joints which arecountersunk for surface flushness.

The pin is shaped and adapted to preload the joint ma- RelatedApphcatlon Data terial surrounding the countersink to create a prede-[62] of 197O- termined residual stress pattern in that region forimprovement of fatigue life. Preferred embodiments of a straight shankbolt, a tapered shank bolt, and a rivet [52] US. Cl 52/758 F, 29/522 arepresented Each pin embodiment has a 70s Conical [51] Int. Cl. F16! 5/02,F161 5/05 head and a Concave transition portion of Specified [58] heldof Search 5? 758 758 dius and smoothness interconnecting the head with l9 29/522 526 the shank. The 70 head and transition portion cooperate toconformably deform and preload the counter- [56] References cued sunkregion of the hole, which is preferably provided UNITED STATES PATENTSwith a convex surface of specified radius at the base of 3,034,61 15/1962 Zenzic 85/1 R X the countersink, and a conical surface having agreater 3,369,440 2/1968 King 85/9 R cone angle than that of the pin.The straight shank 3,391,449 7/l 68 B es 8 D X bolt embodiment furtherdiscloses a hardened convex 3,512,446 5/l970 SfilkhOl'l i 85/9 R lead inportion of specified hape and smoothness ex- 3,561,102 2/197! Diemer52/758 C X tending between the shank and the threads to prevent gallingof the hole during installation of the bolt.

.6 laim? prayinsF ur s PATENTEDJUL 2 m4 3.821.871

saw a or 2 Pm? ART ,4 200 PAW? ART B 202 PR/OX? ART C m 204 H6. 3 30LT(B=/009 R 20 F/6. 3 B0n65= 82') m 206 F/6. fll ET I I w I l g CYCLES x/000 COMPARATIVE FAT/60E L/FE RANGE F01? 2024/ D06BONE TYPE SPEC/MEMS' 1FATIGUE RESISTANT FASTENERS This is a division of application Ser. No.61,153, filed Aug. 5, 1970, now US. Pat. No. 3,748,948, dated July 31,1973.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to fatigue resistant fastener joints for aircraft structures;and, more particularly, to a fastener pin which is shaped to create apredetermined prestressed condition in the fastened material, and to amethod of construction and a joint which give optimum results with thepin.

2. Discussion of the Prior Art At present major design and test effortsare underway in the aircraft industry to develop fastening techniquesand hardware to improve the fatigue life of structural joints subjectedto repetitive loadings. It has long been recognized that most fatiguecracks will develop in the vicinity of fastener holes (see,.for example,Preventing Fatigue Failures, Assembly Engineering, p. 28, May, 1968).Certain innovations, such as holes filled with inteference fit oroversized fasteners, have shown fatigue life improvement overincompletely filled or open holes. An example of this type of fastenerin widespread use today is found in US. Pat. No. 3,034,61 l to ZENZIC,issued May 15, 1962, in which a tapered shank is used to create aninterference fit in a tapered hole of correspondingly smaller (0.0010.0045 inches) diameter. Straight shank interference fit fasteners havebeen less widely used than tapered shank fasteners because uponinstallation they have demonstrated a tendency to gall the walls of thehole, thereby creating minute stress raisers which inherently causeearly fatigue failures. (See, for example, Interference Fasteners forFatigue-Life Improvement, Experimental Mechanics, August, 1965, p. 19A.)

The increased fatigue life of interference fit fastener installations isprimarily attributable to the establishment of a residual stress fieldor prestressed condition in the material immediately surrounding theoversized shank of the fastener pin. Such a prestressed condition tendsto reduce the mean fatigue stress level and/or the alternating stressamplitude, each of which is a primary parameter in fatigue lifetimepredictions.

In flush head fastener installations it has been the usual practice touse a fastener head having a 100 conical surface which matches acorresponding 100 countersunk surface in the material. In testsconducted by the inventor, it has been observed that these countersunkflush head installations often develop fatigue cracks earlier thanequivalent protruding head fastener joints. This phenomenon has appearedin both tapered shank and straight shank fasteners even under closelycontrolled test conditions wherein a prescribed optimum degree of shankinterference is achieved. The cracks most often appear to emanatedirectly from the area of the countersink. This is believed to resultfrom the fact that little attention has been paid to establishing aresidual stress pattern in the area of the countersink comparable tothat existing in the shank area.

In a standard 100 matching cone angle installation, the fastener isnormally provided with a standard minimum fillet radius between theconical head and the shank. The countersunk hole is left sharp at theintersection of the countersink surface with the shank ac- 2 commodatingportion of the hole. In such installations the wide. 100 cone angle willproduce little, if any, preload in a radial direction over the length ofthe countersink, even when the fastener is forced against thecountersink surface and held in position. Such a preload is not at allcomparable to the radial preload induced in the shank area by aninterference fit of the type previously discussed.

It is a primary object of this invention to provide a novel flush headfastener pin for use in a countersunk joint wherein the pin is shapedand adapted to preload the material surrounding the countersink tocreate a prestressed condition which will improve the fatigue life ofthe joint.

A related object of this invention is to teach the use of a transitionportion between the head and the shank of a flush head fastener whichhas a concave surface of specified curvature and smoothness for thepurpose of predictably conformably deforming the countersink material ofthe hole to establish a prestressed condition equivalent to thatobtained around the shank of an interference fit fastener.

A further related object of this invention is to provide a flush headfastener pin having a conical surface with an included cone angle ofapproximately which coacts with a specified transition portion betweenthe head and the shank to establish a prestressed condition around acountersunk hole which preferably is provided with a cone angledifferent from the cone angle of the pin.

A further related object is to teach a method of manufacturing acountersunk flush joint in which a predetermined residual stress patternis established in the material surrounding the countersink by virtue ofphysical interference between a concave transition portion of specifiedradius on the fastenerpin and a convex surface of lesser specifiedradius at the base of the countersunk portion of the hole.

A further objectof this invention is to provide a close tolerance bolthaving a specified lead-in portion between the shank and the threadedend for the purpose of preventing galling of the wall of an interferencefit hole during installation of the bolt,

SUMMARY The above objectives have been achieved in the preferredembodiments of this invention by the provision of a novel fastener pinhaving a conical flush head with a given included cone angle ofapproximately 70, and a transition portion interconnecting thehead andthe shank which comprises a hardened concave surface having a specifiedradius of curvature and smoothness. The transition portion is shaped andadapted to contact and conformably radially expand the inner portion ofthe countersink, which is preferably provided with a specified convexsurface and a cone angle different from that of the pin. Uponinstallation, these features will create a predetermined residual stressdistribution in the countersink region. Two embodiments of a flush headbolt are presented; one having a straight shank and the other a taperedshank. The straight shank version is provided with a cold rolled lead-inportion, be-

tween the shank and the threaded end, tangent with the shank and havinga radius of curvature in a range which is optimum for cold working anddeforming the walls of the hole during installation of the fastener. Arivet embodiment incorporating the 70 head and coacting transitionportion is also presented.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically depicts thestress distribution and interference pattern of typical prior art 100countersink installations.

FIG. 2 is similar to FIG. 1 but is directed to Applicants 70 headfastener and illustrates the stress distribution which can be achievedwhen practicing this invention.

FIG. 3 is a detailed section view of Applicants straight shankinterference fit bolt partially installed in a countersunk hole.

FIG. 4 corresponds to FIG. 3, but shows the bolt fully seated in thecountersunk hole.

FIG. 5 shows a 70 head tapered shank bolt embodiment fully installed ina countersunk hole.

FIG. 6 illustrates a 70 flush head rivet embodimentinstalled in acountersunk hole.

FIG. 7 is a plot of fatigue life test data comparing prior artconfigurations with Applicants fastener installations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 schematically depicts atypical prior art 100 conical head interference fit fastener 10installed in a matching 100 conical countersink, thereby connecting anoverlapping pair of structural members 12 and 14. The dotted linesrepresent the original walls of the hole which have been radiallyexpanded to the solid line positions by the shank of the fastener 10. Asis customary in such installations, the original hole is countersunk toestablish a conical surface 16 which makes a sharp intersection 18 withthe wall 20 of a shank accomodating portion of the hole. The fastener 10is forced into the hole to expand and preload its wall until apredetermined residual stress condition is established. The conical headof the fastener is seated against the matching conical surface 16 of thecountersink. When this is accomplished, a residual tensile stress fieldof the type schematically represented by the force diagram 22 will beestablished. The residual stress level distribution as shown by diagram22 remains relatively constant in the shank region, but falls offsharply in the countersink region above intersection 18. The stresslevel can be seen to fall to a zero level in the countersink region nearthe face of the member 12.

FIG. 2 schematically depicts a 70 head installation constructedaccording to the teachings of this disclosure which connects a pair ofoverlapping structural members 32 and 34. The dotted lines represent theoriginal walls of the hole which have been expanded radially to thesolid line positions established by the shank and head of the fastener30. As will become more apparent in the discussion which follows, aconvex surface 36 is established'at the base of the countersink tointerconnect conical surface 38 with wall 40 of the shank accommodatingportion of the hole. The conical surface 38 is preferably given asomewhat greater included cone angle than that of the head of the bolt30. The fastener head conical surface has an included cone angle of 70and is connected to the fastener shank by a hardened transition portionhaving an enlarged minimum radius of curvature R and specifiedsmoothness.

As the fastener 30 is forced into an interference fit with the walls ofthe hole and the countersink, the transition portion of the bolt willconformably deform the countersink wall so as to induce a residualstress field of the type schematically represented by the force diagram42. In contradistinction to the prior art installation of FIG. 1, thestress level 44 in the shank region can be substantially duplicated bythe level 46 existing over the countersink and extending to the surfaceregion of member 32.

Referring now to FIG. 3, a preferred embodiment of a-straight shank bolt50 is shown in detail. The bolt 50 comprises an externally threadedfastening end means 52, a circular shank 54 having a close tolerancediameter D a circular flush head 56 which includes a convex domed end 58and a conical surface 60 having a given included cone angle A. The shank54 is connected to head 56 by a transition portion 62, comprising a coldrolled smooth concave surface having an enlarged nominal radius R, whichis arranged and shaped to conformably deform and prestress the walls ofa countersunk surface upon installation of the bolt 50. The fasteningend means 52 is connected to shank 54 by a lead-in portion 64 comprisingconvex surface means with a specified radius R for coldworking andexpanding the surface of the wall of the shank accommodating portion ofthe hole during installation of the bolt. The included cone angle A ofthe conical surface 60 is preferably established at approximately 70.The 70 angle has been selected in view of the following factors: l) awide cone angle such as 100 which is prevalent in prior artinstallations offers very little wedge action of the type contemplatedby this invention; (2) decreasing the cone angle will increase thestress concentration at the intersection of the conical surface and theouter surface of the member; (3) decreasing the cone angle reduces theseated bearing area and hence in a low strength material will reduce theallowable tensile load which can exist in the fastener; and of primaryimportance based on test results, (4) the 70 head appears to interactand cooperate most favorably with the novel enlarged radius transitionportion 62 in providing a means for predictably displacing the materialsurrounding the countersink portion of the hole to obtain asubstantially uniform residual stress distribution over the countersinkregion. When an enlarged radius transition portion such as contemplatedby this invention is used with a fastener head having a cone anglewithin the range of from about to 85, a residual stress pattern givingimproved fatigue life will result, even in a conventionally countersunkhole. However, a nominal value of approximately appears to offer thebest stress level distribution, especially when the countersunk hole isprepared in the manner to be described.

The transition portion 62 of the bolt 50 is preferably manufactured by acold rolling process to obtain a hardened surface having smoothnesscharacteristics superior to a surface finish of 32 Root Height Ratio(RHR) as defined by US. Government Military Specification MIL-STD-IO,with an increased smoothness of approximately 16 RHR being preferred.The concave surface of transition portion 62 performs in the manner of ametal forming tool or die, to conformably deform found to give optimumresults. For small diameter bolts, a nominal radius of curvature of0.060 inches with a tolerance of $0.010 inches has been found to performsatisfactorily without galling or other damage when used in combinationwith a 70 cone angle surface on the bolt. The bolt should be constructedof a high-strength material which can be hardened by cold working andwhich has an ultimate shear strength in excess of 85,000 psi withcorrespondingly high surface bearing properties. Fasteners constructedof 6Al-4V and Beta 3 titanium have been used in combination withaluminum structural members in most of the fatigue tests conducted byApplicant. Titanium or high strength steel fasteners should be used whenthe structural members to be joined are constructed of titanium.

The lead-in portion 64 which interconnects the bolt shank 54 and thefastening end means 52 comprises a circular convex surface of revolutionprecisely tangent to shank 54 and having a maximum diameter smaller thanthe minimum diameter of the shank. Lead-in portion 64 should bemanufactured by cold rolling to a smoothness of less than 32 RHR(preferably less than 16 RHR) and formed to a nominal radius ofcurvature R which falls within the range of from 20% to 60% of the shankdiameter D For 3/16, A and 5/16 inch diameter bolts, the nominal radiusused by the inventor has been standardized at 0.090 inches with atolerance of i0.015 inches. In the past, interference fit bolts of thesediameters have been proposed which incorporate either a minimal radius(0.030 inches), an enlarged radius (approximately 70% of the shankdiameter), or a slight taper in the lead-in region. However, mocroscopicanalyses performed by the inventor indicate that when these bolts areused in high interference fit joints,

the minimal radius tends to gall the hole by a cutting action, and theenlarged radius or the taper tends to gall by collection of minute chipparticles in the small acute angle formed between the bolt lead-insurface and the walls of the hole. A bolt configuration has also beenproposed which utilizes a bead of a larger diameter than the adjacentshank diameter to cold work the walls of the hole and reduce the forcerequired to install the bolt. However, this beaded bolt inherently tendsto create extremely high localized bearing pressures which could causegalling problems under certainconditions. Also, the bead configurationsrelies upon spring back of the hole wall as the bead passes through thehole and hence inherently requires more disturbance of the walls of thehole than does Applicants lead-in portion 64.

Applicants bolt 50 is also provided with a crown end portion 58 having aconvex surface with a minimum radius of curvature exceeding twice theshank diameter D Although exaggerated for purposes of illustration, theradius is preferably selected such that the crown rises only 0.002 to0.006 inches above a conventional flat head. Aerodynamic drag testingand analyses indicate improved results for this configuration when usedon external surfaces, particularly when adverse tolerance conditions areconsidered. Also, of particular importance to high interference fitjoints which require high driving forces, the convex crown curvaturecompensates for off-center driving forces, and serves to prevent damagecaused by impact of the driving tool on the surface of the structuralmember.

FIG. 3 shows the bolt 50 partially installed in a hole through anoverlapping pair of workpieces 72 and 74. A wall 76 for hole 70 isdrilled to a closely controlled diameter, D as shown. A countersinkconical surface 78, having a given cone angle B which is preferably atleast 4 larger than bolt cone angle A, is machined in the surface ofmember 72. The wall 76 and the inner portion of the countersink conicalsurface 78 are interconnected by a convex surface 80 having a minimumradius of curvature r, ofat least 0.015 inches, and preferably of anominal value of 0.030 inches with a tolerance of 310.005 inches. Asnoted earlier, the in tersection of the conical countersink surface withthe wall of the hole has normally been left sharp in prior artinstallations.

In FIG. 3, the lead-in portion 64 can be seen to be cold working andexpanding the wall 76 as the bolt 50 is forced into the hole. Thestraight shank fastener of this invention was developed for relativelynew and untried straight shank high interference fit installations (D Dby 0.003 0.006 inches) in order to obtain higher level residual stresspatterns in the shank region for improved fatigue life. With these highinterferences, a larger driving force is required to install a straightshank fastener, and the risk of galling the hole increases. For thesereasons, most prior art joints having high interference fits have usedtapered shank fasteners which inherently involve higher costs. However,Applicants straight-shank lead-in portion 64 has demonstratedsatisfactory results'in joints having interference fits of up to 0.009inches. Lead-in convex surface means such as here disclosed may ofcourse be used with protruding head bolts as well as the flush headconfiguration shown.

FIG. 4 shows Applicants flush head bolt 50 secured by an internallythreaded nut means in a seated position in the countersink. The dottedlines 76, 78 and 80 indicate the original undeformed walls of the holewhich have been expanded to the position of the corresponding solid linebolt surfaces 54, 62 and 60. Although exaggerated for purpose ofillustration, the difference between the dotted line and solid linepositions is indicative of the type of material deformation and coldworking imparted to the finished joint. Results similar to the residualstress diagram 42 of FIG. 2 have been obtained in joints using A and 5/l 6 inch diameter bolts and the following geometric parameters: (1) abolt head cone angle A of 70; (2) a bolt transition portion 62 nominalradius R, of 0.060 inches; (3) a shank diameter interference of from0.003 to 0.006 inches; (4) a countersink hole cone angle B of 82; (5) acountersink convex surface radius r of 0.030 inches; and (6) a lead-inportion 64 nominal radius R of 0.090 inches.

In larger sizes inch and above) requiring high driving forces,countersink cone angles as low as 75 have been used with a 70 bolt toproduce improved joints from a fatigue standpoint. It will becomeevident to those skilled in this art that various trade-offs areavailable in terms of the bolt radius R the countersink radius r,, thecone angles A and B, and the degree of interference between D, and D inorder to tailor a residual stress distribution extending through thecountersink region of the hole. If the difference in the cone angles Aand B falls below about 4, the interference caused thereby will probablybe minimal in terms of influence on the residual stress pattern, whichwould then be dominated by the interference due to the enlarged radiusR; operating on the countersink convex surface with smaller radius rFrom FIGS. 3 and 4 it can be seen that the material surrounding thecountersink has been acted upon and deformed outwardly, first by theconcave surface 62 and then by that surface acting in cooperation withthe 70 conical surface 60, as the bolt is forced into a seated position.The conical surface 60 serves to conformably guide the flow of materialby a wedging action which assists in preloading the upper portion of thehole. A slight protuberance 92 is usually observable around the hole atthe outer surface of the countersink. This is indicative that theinduced residual stress distribution extends to the surface of thematerial in the manner desired.

A chronic problem in flush head countersunk fastener installations hasbeen that of end grain corrosion due to exposure to the atmosphere ofthe outer portion of the hole above the conical surface of the bolt.This exposure is observable in the prior art installation of FIG. 1, andresults from theprovision of the flat longitudinally extending surfacebetween the conical surface of the bolt head and its-end portion. Suchflat surfaces are customarily provided in order to produce a concentricfastener with a clean appearance when trimming the sharp edge at thislocation. In the FIG. 4 embodiment, it can be seen that Applicant hasminimized this problem by requiring that the trimmed surface (at 94) beleft relatively sharp; with a maximum longitudinal dimension of lessthan 0.005 inches, and by sizing of the bolt head and countersunk holefor an appropriate fit after installation.

Referring now to FIG. 5, a tapered bolt embodiment of Applicants flushhead countersunk fastener is shown. As is noted in the ZENZIC patentreferenced earlier, the bolt 100 is customarily given a uniform taper of0.020 inch per inch along it shank. A matching hole 101 is provided witha tapered wall having the same slope but correspondingly smallerdiameters by a predetermined amount, usually from 0.001 to 0.0045 inchinterference. The FIG. 5 tapered bolt 100 is similar to the straightshank embodiment in having a threaded fastening end means 102, acircular shank 104 with a predetermined maximum diameter D a circularflush head 106 including a convex domed end 108 and a conicalsurface 110having a given included cone angle A which is preferably approximately70. The shank 104 is connected to head 106 by a transition portion 112,comprising a smooth concave surface having an enlarged radius R which ispreferably ofa value greater than of the predetermined maximum di ameterD In terms of function and the resulting residual stress distribution inthe countersink region, the results obtained will be similar to thosediscussed in connection with the straight shank embodiment. A lead-inportion 114 having a convex surface similar to surface 64 of FIG. 3 canbe advantageously incorporated in the tapered shank embodiment toprevent galling during installation, an occasional problem in the lowerregions of the tapered hole in certain high interference fit cases.

FIG. 6 illustrates a flush head rivet 120 constructed according to theteachings of this disclosure to establish a predetermined residualstress pattern in the countersink region for fatigue life improvement. Amaterial such as Beta ll titanium is preferably used for the rivet pinmaterial. As discussed in connection with previous embodiments, atransition portion 126 having a concave surface of enlarged radius isused to conformably deform and preload the material surrounding thecountersink as the rivet is driven into the hole. The shank diameter D,in this case need not be sized for an interference fit with the originalunstressed hole. As is the usual case in rivet installations,interference can be achieved with expansion of the shank and conicalhead during swaging of the upset end of the rivet. The conical headportion of the rivet will tend to swell in the countersink, therebypotentially reducing or eliminating the need for a difference in coneangles A and B prior to installation. While a difference in cone anglesis recommended, it may be possible with certain rivets, or otherradially expanding'fasteners, to markedly reduce or even eliminate thisdifference, depending upon the residual stress distribution desired.Conventional driving tools can be used for the FIG. 6 installation,although the required driving force will be higher, dependent upon themagnitude of the prestress induced in the countersunk region.

Referring now to FIG. 7, fatigue test results for three jointsconfigured according to this invention are contrasted with comparableprior art joints in graphic form. At least five dogbone type specimenswere identically constructed of 2024 aluminum for each type of joint.Conditions were maintained as consistent as possible during all testing,the horizontal width of each plot representing the extent of datascatter obtained. The three upper plots, labeled 200, 202 and 204, werederived from data taken from the following prior art configurations: (A)a l00 flush head tapered bolt in a matching conventional 100 countersunkhole; (B) a beaded" 100 flush head straight shank bolt in a matching 100hole; and (C) a conventional 100 rivet in a matching 100 hole. The threelower plots, labeled 206, 208 and 210, were derived from data taken fromjoints constructed according to this disclosure as follows: (l) a FIG. 3flush head bolt in a countersunk hole with cone angle B (2) a FIG. 3 70flush head bolt in a countersunk hole with cone angle B 82; and (3) a 70flush head rivet installed substantially as shown in FIG. 6.

The FIG. 7 data plots graphically reflect the magnitude of theimprovement in fatigue life which may be obtained using fastener pinsconstructed according to this disclosure. For example, Applicants rivetconfiguration (plot 210) shows a mean fatigue life improvement ofapproximately 100% over the prior art rivet (plot 204).

The invention has been described and illustrated in detail using certainpreferred embodiments and dimensioning developed by Applicant to date.Many variations and modifications will occur to persons skilled in thisart without departing from the spirit and scope of the invention.Accordingly, it is intended in the appended claims to cover all suchvariations and modifications.

What is claimed and desired to be secured by US. letters Patent is:

l. A flush structural joint for transferring load between an overlappingpair of structural members comprising: walls on said members forming aconcentric hole passing therethrough, said walls including a circularshank accommodating portion extending from an outer surface of one ofsaid members and a concentric countersink portion of increased diameterwhich extends to the opposite outer surface of the other of saidmembers, said countersink having an outer portion adjacent the outersurface of the other of said members, and an inner portion of reduceddiameter adjacent to said shank accommodating portion, said innerportion including a convex surface extending to and tangent with thewalls of said shank accommodating portion, said convex surface having aminimum radius of curvature of at least 0.015 inches; a fastener forcreating an interference fit against said inner portion of saidcountersink comprising: a circular shank which fits into said shankaccommodating portion of said hole, said shank having a predeterminedmaximum diameter, a circular flush head connected to one end of saidshank and nested within said countersink, a transition portioninterconnecting said head and said shank comprising a concavely curvedsurface having a minimum radius of curvature of a value greater than ofsaid predetermined maximum diameter, a fastening end section connectedto'the opposite end of said shank from said flush head and protrudingout of said hole to allow a securing load to be applied to said membersto retain the fastener in said hole; wherein said transition portion isforced into the convex surface of said inner portion of said countersinkto radially expand and preload the material surrounding the countersinkto cause a prestressed condition to exist therein which tends to preventfatigue cracks from developing around said countersink when said membersare subject to repetitive loadings.

2. The joint of claim 1 wherein said circular flush head comprises aconical surface having a selected given cone angle within the range offrom 60 to 85, said conical surface of said head being arranged to bearagainst a conical surface of the countersink to thereby radially expandand preload the material surrounding 10 the countersink when said flushhead is forced into position in said countersink.

3. The joint of claim 2 wherein said conical surface of said countersinkhas an initial unstressed cone angle which is at least 4 different fromsaid selected given cone angle of said circular flush head.

4. The joint of claim 1 wherein said fastening end section comprises anexternally'threaded section having a thread major diameter smaller thanthe minimum diameter of said shank, and wherein the joint includes aninternally threaded nut means for engagement with said threaded sectionto retain the head of the fastener in a seating engagement with saidcountersink by tightening of said nut means.

5. The joint of claim 4 wherein said shank is tapered approximately0.020 inch per inch along the entire length of said shank and saidpredetermined maximum diameter defines the larger end of said shanklocated adjacent said transition portion, the diameters of the wall ofthe tapered shank being greater than the initial corresponding diametersof said hole by at least 0.001 inches to thereby expand and preload thematerial surrounding said shank accommodating portion of said hole tocause a tensile prestress condition to exist therein which tends toprevent fatigue cracks from developing around said shank accommodatingportion of said hole.

6. The joint of claim 4 wherein said fastener further comprises alead-in portion for cold working the shank accommodating portion of thewalls of said hole during insertion of said fastener, said lead-inportion comprismg:

a convex surface having a radius adjoining and tangent to said shankwhich has a value within the range of from 20% to 60 of saidpredetermined maximum diameter.

1. A flush structural joint for transferring load between an overlappingpair of structural members comprising: walls on said members forming aconcentric hole passing therethrough, said walls including a circularshank accommodating portion extending from an outer surface of one ofsaid members and a concentric countersink portion of increased diameterwhich extends to the opposite outer surface of the other of saidmembers, said countersink having an outer portion adjacent the outersurface of the other of said members, and an inner portion of reduceddiameter adjacent to said shank accommodating portion, said innerportion including a convex surface extending to and tangent with thewalls of said shank accommodating portion, said convex surface having aminimum radius of curvature of at least 0.015 inches; a fastener forcreating an interference fit against said inner portion of saidcountersink comprising: a circular shank which fits into said shankaccommodating portion of said hole, said shank having a predeterminedmaximum diameter, a circular flush head connected to one end of saidshank and nested within said countersink, a transition portioninterconnecting said head and said shank comprising a concavely curvedsurface having a minimum radius of curvature of a value greater than 15%of said predetermined maximum diameter, a fastening end sectionconnected to the opposite end of said shank from said flush head andprotruding out of said hole to allow a securing load to be applied tosaid members to retain the fastener in said hole; wherein saidtransition portion is forced into the convex surface of said innerportion of said countersink to radially expand and preload the materialsurrounding the countersink to cause a prestressed condition to existtherein which tends to prevent fatigue cracks from developing aroundsaid countersink when said members are subject to repetitive loadings.2. The joint of claim 1 wherein said circular flush head comprises aconical surface having a selected given cone angle within the range offrom 60* to 85*, said conical surface of said head being arranged tobear against a conical surface of the countersink to thereby radiallyexpand and preload the material surrounding the countersink when saidflush head is forced into position in said countersink.
 3. The joint ofclaim 2 wherein said conical surface of said countersink has an initialunstressed cone angle which is at least 4* different from said selectedgiven cone angle of said circular flush head.
 4. The joint of claim 1wherein said fastening end section comprises an externally threadedsection having a thread major diameter smaller than the minimum diameterof said shank, and wherein the joint includes an internally threaded nutmeans for engagement with said threaded section to retain the head ofthe fastener in a seating engagement with said countersink by tighteningof said nut means.
 5. The joint of claim 4 wherein said shank is taperedapproximately 0.020 inch per inch along the entire length of said shankand said predetermined maximum diameter defines the larger end of saidshank located adjacent said transition portion, the diameters of thewall of the tapered shank being greater than the initial correspondingdiameters of said hole by at least 0.001 inches to thereby expand andpreload the material surrounding said shank accommodating portion ofsaid hole to cause a tensile prestress condition to exist therein whichtends to prevent fatigue cracks from developing around said shankaccommodating portion of said hole.
 6. The joint of claim 4 wherein saidfastener further comprises a lead-in portion for cold working the shankaccommodating portion of the walls of said hole during insertion of saidfastener, said lead-in portion comprising: a convex surface having aradius adjoining and tangent to said shank which has a value within therange of from 20% to 60 % of said predetermined maximum diameter.